“The Fifth Heart,” by Dan Simmons. (Little, Brown)

Is there a contemporary writer more at home in a multitude of genres — not to mention geographical locales and historical eras — than Dan Simmons? Since the appearance of his first novel, “Song of Kali,” in 1985, Simmons has written award-winning horror (“Carrion Comfort”) and first-rate science fiction (“The Hyperion Cantos”), along with such unclassifiable works as “Phases of Gravity” and “The Hollow Man,” with its alternating echoes of Dante and T.S. Eliot. More recently, he has published a series of ambitious historical thrillers — all of them deeply researched and thoroughly entertaining — ranging from Everest to the Arctic to the American West. His latest, “The Fifth Heart,” falls generally into this latter category, but is not quite like anything that has come before.

At first glance, “The Fifth Heart” appears to be yet another Sherlock Holmes pastiche, and there has never been a shortage of those. This one, however, manages to be different. The story takes place not in fog-shrouded London, but in the Gilded Age America of 1893. The great detective’s partner in this extravagant adventure is not the stolid Dr. Watson, but that most refined and fastidious of American novelists, Henry James. The contrast between these vastly different personalities lends both tension and occasional humor to this most un-Jamesian narrative.

The story begins in Paris, where a despondent James, his personal and professional lives in disarray, is preparing to throw himself into the Seine. He is saved by the presence of another would-be suicide, a man he recognizes as Sherlock Holmes. Holmes has a unique reason for wanting to die: He has deduced that he may not be a real person, but a fiction, “some ink-stained scribbler’s creation.” This existential dilemma permeates the narrative, adding metaphysical speculation to the assorted mysteries that will follow.

In an uncharacteristically awkward transition, a revitalized Holmes whisks James off to a nearby cafe, where he proceeds to embroil the writer in a pair of interconnected cases. The first concerns the 1885 suicide of Clover Adams, wife of the American historian Henry Adams and founding member of an elite social circle called The Five Hearts. Every year, on the anniversary of Clover’s death, both Holmes and the remaining members have received a card bearing a single typed message: “She was murdered.” With the aid of James’s limitless social connections, Holmes enters the closed circle of the Five Hearts and begins to investigate.

The second, much larger case involves, quite literally, the fate of nations. Holmes has come to America to foil a vast conspiracy of anarchists and arch-criminals whose plans, if successful, will change the political face of the Western world. The ensuing adventure takes Holmes and James from the heights of American society to the lower depths, and from Washington, D.C., to New York City to the “White City” of Chicago, where the World’s Fair is about to begin.

The combination of Holmes’s existential speculations with the complex mechanics of two intertwined investigations adds richness and complexity to what might otherwise have been a standard post-Doyle Sherlock Holmes tale. An added virtue is the novel’s re-creation of late 19th-century America. Simmons has always brought a powerful sense of place to his fiction, and in “The Fifth Heart” he makes every element, from the stately homes of Washington to the White City at the heart of the World’s Fair, a palpable presence.

Finally, it is the presentation of the two central characters that most engages our attention and affection. Simmons’s version of Holmes is both instantly recognizable and subtly different. This Holmes is a man uncomfortably caught between his “real” existence and his shadow life as hero of a series of popular fictions. The result is a much more human portrait of the Great Detective than we usually encounter.

The decision to use Henry James as a substitute Watson seems odd at first, but in the end works wonderfully well. The James of the novel is an unlikely, thoroughly appealing hero. He is a lonely, brilliant, vulnerable man of uncertain sexuality who is married only to his art. Though he doesn’t know it, he is in need of a spiritual awakening, one that arrives through the unexpected intervention of Sherlock Holmes. Watching this odd pair progress from the suicidal thoughts of the opening pages to a renewed sense of purpose is one of the central pleasures of this beguiling book.

Bill Sheehan is the author of “At the Foot of the Story Tree: An Inquiry into the Fiction of Peter Straub.”

THE FIFTH HEART

By Dan Simmons

Little, Brown. 617 pp. $28